The present invention relates to a novel developer for a presensitized plate for use in making a lithographic printing plate (hereinafter possibly referred to as xe2x80x9cPS platexe2x80x9d) and a method for preparing a lithographic printing plate. More specifically, the present invention relates to a developer optimum for developing a photopolymerizable presensitized plate for use in making a lithographic printing plate (hereinafter possibly referred to as xe2x80x9cphotopolymerizable PS platexe2x80x9d), which never shows any reduction of developing characteristics due to the elapse of a long period of time and repeated use, can ensure a good ability to develop the non-image area (non-exposed area) of the lithographic printing plate, does not damage the image area (exposed area) of the plate due to the development, ensures the formation of firm images and allows the achievement of high printing durability as well as a method for preparing a lithographic printing plate.
The developers widely used conventionally for developing a photopolymerizable PS plate may roughly be divided into the following three groups: a) non-aqueous developers mainly comprising organic solvents; b) aqueous developers mainly comprising inorganic alkalis; and c) aqueous developers mainly comprising organic bases.
Among these, those presently used include, for instance, aqueous developers b) and c) at the request of the environmental protection.
Characteristic properties of these two developers will be detailed below. The inorganic alkaline developer b) is characterized in that it usually comprises a silicate and has a pH value in the proximity to 12 in order to hydrophilize the surface of a substrate of the printing plate after the development.
This silicate component is essential for the subsequent hydrophilization treatment or for preventing the non-image area from being contaminated by ink during printing, i.e., for the prevention of scumming.
For instance, there have been known a developer having a pH value of not less than 12 such as those disclosed in Japanese Un-Examined Patent Publication (hereunder referred to as xe2x80x9cJ.P. KOKAIxe2x80x9d) No. Hei 8-248643 and a developer having a pH value of not more than 12 such as those disclosed in J.P. KOKAI No. Hei 11-65129. However, the former or the developer having a pH value of not less than 12 is liable to dissolve aluminum currently used as a substrate for the printing plate and, in particular, in case of a dot (small dot) occupying a small image area, it may dissolve the aluminum substrate immediately below the image area due to the so-called side etching phenomenon and this results in a phenomenon in which small dots are removed from the substrate (skipping of small dots) during printing. In other words, this developer suffers from such a problem that it may significantly deteriorate the printing durability of the resulting printing plate.
Moreover, the latter or the developer having a pH value of not more than 12 is excellent in the foregoing printing durability and an ability of preventing any contamination of printed matter, but it is liable to cause reduction of the pH value when continuing the developing treatment over a long period of time due to, for instance, the effect of carbon dioxide present in the air and the developer causes separation of a silicate during the development and this in turn leads to the occurrence of such a novel problem that it never allows any stable development.
In addition to the foregoing, J.P. KOKAI Nos. Sho 61-109052, 2000-81711 and Hei 11-65126 and West German Patent No. 1,984,605 disclose developers free of any silicate, but all of them were not only inferior in the prevention of any contamination of printed matter to those comprising silicates, but also could simultaneously satisfy the requirements for the printing durability and the prevention of any contamination of printed matter, only with great difficulty.
On the other hand, there have been known, as the organic base-containing developer c), for instance, those each comprising an organic amine such as ethanolamine and an alcoholic organic solvent such as benzyl alcohol as an auxiliary developer. In this case, however, such a developer undoubtedly has a low pH value on the order of 10 and therefore, it is not susceptible to the influence of carbon dioxide and is excellent in the processing stability. However, it also suffers from problems in that it is not only disadvantageous in the hydrophilization of the substrate, but also has an extremely high ability of penetration into the image area, that it adversely affects, in particular, small dots and that it is accompanied by the deterioration of the image-forming ability or such a phenomenon that small dots are removed from the substrate during development.
In other words, under the existing circumstances, there is not proposed any developer suitably used for developing the photopolymerizable PS plate, while taking into consideration, for instance, the image-forming ability, prevention of any scumming and printing durability as well as the processing stability.
Regarding the composition of the developer, it has been known that factors such as the presence of a silicate, the level of pH and the presence of an inorganic or organic alkali may exert a great influence on the developing phenomenon, but any combination thereof conventionally proposed has never permitted the solution of the foregoing problems at all.
Moreover, in case of the photopolymerizable PS plate, when continuing the developing treatment over a long period of time, insolubles are accumulated, agglomerated and settled down in the developer to thus form developing sludge and this may make the development treatment unstable.
Components constituting such developing sludge have not yet been clearly elucidated. As a result of the analysis thereof, however, most of them are found to be components included in a light-sensitive layer of a PS plate and insoluble in the developer.
As such developer-insoluble components, there may be listed, for instance, a variety of coloring agents added to the light-sensitive layer for distinguishing the exposed area from the non-exposed area. These coloring agents used in the photopolymerizable PS plate are so-called pigments, which are dispersed in the light-sensitive layer in the form of aggregates in which they maintain their crystalline conditions rather than in a molecularly dispersed state, for the purpose of inhibiting any desensitization due to the capture of radicals or unnecessary interaction (such as energy transfer or electron migration) with an initiator system (for instance, an initiator alone or a combination of a sensitizing dye and an initiator). However, such a pigment is essentially insoluble in a developer. Therefore, it is only temporarily dispersed in the developer in the initial stage of the developing step for removing the non-exposed area, but when continuing the developing treatment over a long period of time, it is accumulated, agglomerated and settled down in the developer to thus form developing sludge.
On the other hand, a variety of photopolymerization initiators (radical generators) have been incorporated into the photopolymerizable PS plate for coping with the recently progressed laser light sources and among them, titanocene type initiators have been known as initiators, which are excellent in stability and sensitivity and which are light-sensitive in the wavelength ranges of laser light sources. However, this titanocene type initiator is an organic metal and thus insoluble in the developer like the foregoing pigments as coloring agents. Therefore, it is temporarily dispersed in the developer, but when continuing the developing treatment over a long period of time, it is also accumulated, agglomerated and settled down in the developer to thus form developing sludge.
Regarding the processing stability, it is quite critical to solve not only the foregoing problem of the silicate-separation associated with the reduction of pH, but also the problem of the developing sludge-formation.
In other words, there has been desired for the development of a developer for a photopolymerizable PS plate, which can satisfy the requirements for the foregoing image-forming ability, for both the prevention of contamination of printed matter and the printing durability and for the processing stability.
Accordingly, it is an object of the present invention to provide a developer for a PS plate, which can satisfy both the requirements for the prevention of scumming and the printing durability without impairing the image-forming ability, which can dissolve the foregoing developer- insoluble compounds or disperse and stabilize the same over a long period of time and which can improve the processing stability as well as a method for preparing a lithographic printing plate.
The inventors of this invention have conducted various studies and have succeeded in the achievement of the foregoing object of the invention on the basis of the following construction.
More specifically, the present invention relates to (1) a developer for a photopolymerizable presensitized plate for use in making a lithographic printing plate characterized in that it comprises an alkali silicate and a nonionic compound represented by the following general formula (I), it has a molar ratio: SiO2/M2O (wherein M represents an alkali metal or an ammonium group) ranging from 0.75 to 4.0, and a pH value ranging from 11.5 to 12.8:
Axe2x80x94Wxe2x80x83xe2x80x83(I)
(In the formula, A represents a hydrophobic organic group whose logP as determined for Axe2x80x94H is not less than 1.5 and W represents a nonionic hydrophilic organic group whose logP as determined for Wxe2x80x94H is less than 1.0).
According to a preferred embodiment of the present invention, the foregoing developer has a content of an alkali silicate ranging from 0.1 to 3% by weight as expressed in terms of the amount of silicon dioxide (SiO2). According to another preferred embodiment of the present invention, the foregoing developer has a content of nonionic compound ranging from 0.1 to 15% by weight. Preferably, the nonionic compound represented by the foregoing general formula (I) usable herein is at least one member selected from the group consisting of nonionic aromatic ether type surfactants represented by the following general formula (I-A) and nonionic aromatic ether type surfactants represented by the following general formula (I-B). 
(Wherein R1 and R2 each represents H or an alkyl group having 1 to 100 carbon atoms and n and m each represents an integer ranging from 0 to 100, provided that n and m are not simultaneously zero).
According to a preferred embodiment of the present invention, the developer further comprises a chelating agent. According to a further preferred embodiment of the present invention, the developer may comprise carbonic acid or a carbonate.
According to another aspect of the present invention, there is also provided (2) a method for preparing a lithographic printing plate comprising the steps of imagewise exposing, to light, a presensitized plate for use in making a light-sensitive lithographic printing plate, which comprises a substrate provided thereon with a photopolymerizable light-sensitive layer containing a compound having at least one addition-polymerizable ethylenically unsaturated double bond and a photopolymerization initiator; and then developing the exposed presensitized plate using the foregoing developer.
In a preferred embodiment of the method for preparing a lithographic printing plate according to the present invention, the light-sensitive layer has an acid value of not more than 1.0 meq/g and more specifically, the light-sensitive layer comprises a compound having an acid group whose pKa value is not more than 9 and the light-sensitive layer has an acid value ranging from 0.20 to 0.60 meq/g.
In another preferred embodiment of the plate-making method according to the present invention, the imagewise exposed presensitized plate is developed in such a manner that the rate of the development of the non-exposed area is not less than 0.05 xcexcm/s and the rate of the developer penetrating into the exposed area is not more than 0.1 xcexcm/s.
In a further preferred embodiment of the plate-making method according to the present invention, the light-sensitive layer comprises a titanocene type initiator or a coloring agent (or a pigment). As a still further preferred embodiment of the plate-making method according to the present invention, the substrate of the presensitized plate is an anodized aluminum substrate provided thereon with an organic compound carrying a phosphorus atom-containing acidic group or an organic silicone compound.
As has been described above, the present invention has succeeded in the development of a developer, through the use of specific components, which can simultaneously satisfy both the requirements for the prevention of contamination of printed matter and the printing durability of the printing plate without impairing the image-forming ability of the PS plate for the printing plate, which can dissolve the foregoing developer-insoluble compounds or disperse and stabilize the same over a long period of time and which can improve the processing stability.
The developer containing specific components has presently been considered to be a developer composition, which satisfies the following requirements.
First of all, the developer composition should exert a very good effect on the image-forming properties of a PS plate (more specifically, it has a high ability of developing non-exposed areas, while it has a weak permeability into imagewise exposed areas. Moreover, the light-sensitive layer is dissolved without undergoing any swelling thereof and it is successively dissolved from the surface.)
Secondarily, it should be able to completely remove the light-sensitive layer on the non-exposed areas to thus regenerate the substrate surface as a hydrophilic one, which is never accompanied by any contamination of printed matter.
Thirdly, it should comprise a nonionic compound represented by the foregoing general formula (I) having a hydrophobic site capable of undergoing interaction with the foregoing developer-insoluble compounds and a hydrophilic site for dispersing and stabilizing the developer-insoluble compounds in water, in order to stably disperse or solubilize the developer-insoluble compounds.
Fourthly, it contain a chelating agent for the removal of any divalent metal such as Ca ions included in water. The divalent metal may be a factor, which makes the developing treatment unstable.
Regarding the first and second requirements among the foregoing ones, the characteristic properties of the components of the light-sensitive layer also become an important factor. There is not any restriction in case of the light-sensitive layer of, in particular, a photopolymerizable PS plate, but the fact that the acid value of the light-sensitive layer is low as compared with the conventional one is considered to be important for the achievement of a synergistic effect with the developer of the present invention, so far as one can presently know.